This invention relates to control devices for electric discharge machines (hereinafter referred to as "electric discharge machining control devices"), and more particularly to an electric discharge machining control device in which the machining gap between an electrode and a workpiece is suitably controlled by a position control servo system having sampling, logic operation and holding functions.
In general, it is essential that the servo system of an electric discharge machine quickly control the machining gap in response to the electric discharge conditions. Accordingly, a servo actuator such as a hydraulic cylinder, a pulse motor or a DC servo motor has been employed to drive the machining electrode. Furthermore, a position control servo system having sampling, logic operation and holding functions has been used to control the machining gap. A general purpose processing unit such as a microprocessor or microcomputer has been employed for the position control servo system.
FIG. 1 shows one example of an electric discharge machining control device of this type. In this device, a DC servo motor is employed as the actuator. A current-controlled pulse voltage is applied across the discharging gap between an electrode 12 and workpiece 14 by an electric source 16. The pulse voltage, after being converted into a servo signal (down) for decreasing the machining gap and a servo signal (up) for increasing the machining gap according to the respective discharge periods by means of discrimination circuit 18, is applied to a sample value processing unit 20 which has sampling, logic-operation and holding functions.
In the processing device 20, an input latching circuit 22 is caused to sample the above-described servo signal every period of time .DELTA.T. The servo signal thus sampled is converted into a main shaft position instruction signal by a logic operation circuit 24. The instruction value is held by an output latch circuit 26 until the next instruction value is provided. The instruction value is a parallel digital output. The parallel digital output is converted into a series digital output by a binary rate multiplier (BRM) 28 and is then applied, as a count input, to an error counter 30 which forms a position control servo system. In the counter 30, the instruction value mentioned above is reduced by the amount of displacement of the main shaft which has been detected by a main shaft position detector 44. The result of this subtraction, namely, a position error value, is converted into an analog signal by a D/A converter 32. The analog signal is applied, as a speed instruction, to an electrode drive speed control servo system which comprises a speed amplifier 34, a DC servo motor 36 and a specified speed generator 38. That is, the DC servo motor 36 is driven by the analog signal, to turn a ball screw 40, to thereby displace the main shaft 42 to which the electrode 12 has been fixedly secured, until the displacement of the main shaft 42 coincides with the position instruction, thus controlling the machining gap.
The discrimination circuit 18 is substantially similar to that disclosed by U.S. Pat. No. 3,825,715, and operates as described below.
The waveforms of the voltage and current applied to the machining gap in FIG. 1 are as shown in FIGS. 2a and 2b, respectively. As is apparent from FIG. 2, in many cases, electric discharge is not caused immediately after the voltage pulse is applied across the machining gap; in other words, a no-load voltage 100 appears for a certain period of time, and thereafter electric discharge occurs, i.e., a discharge current 102 flows while a voltage 104 is developed across the machining gap. This electric discharge is suspended at the start of a pause time 106.
The provision of the non-load voltage 100 as described above means that the dielectric strength of the machining gap has been sufficiently recovered during the pause time. When electric discharge occurs within a period of time .tau..sub.2, it is determined that the gap length is suitable, and a "stop signal" as shown in FIG. 2d is provided as the servo signal; and when electric discharge occurs within a period of time .tau..sub.3, it is determined that the gap length is excessive, and an "electrode-down-signal" as shown in FIG. 2c is provided. On the other hand, when the no-load voltage is developed for a very short time or when it is not developed at all, the dielectric strength of the machining gap was not recovered during the pause time. Therefore, when electric discharge takes place within the period of time .tau..sub.1 it is determined that the gap length is too short, and an "electrode-up-signal" as shown in FIG. 2e is provided. As is apparent from the above description, the discrimination circuit 18 outputs the servo signal for every pulse voltage.
The conventional discrimination circuit 18 provides a servo signal for each discharge pulse as described above. Therefore, the circuit samples an instantaneous value at the time of sampling as an input means for a sample value control system performing a sampling operation at predetermined intervals, thus disregarding the servo signal between sampling time instants. Therefore, as is known from sampling theory, the discrimination circuit forms a filter whose frequency is about half of the sampling frequency. Accordingly, it is difficult for the conventional discharge machining control device to perform servo control with a good follow-up characteristic.